<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
   <!-- <style>
        #demo {
            width: 100px;
            height: 100px;
            background-color: pink;
        }
    </style>-->
</head>
<body>
 <div id="demo"></div>
 <div class="test"></div>
 <script>
    document.getElementById("demo").style.width = "200px";
    document.getElementById("demo").style.height = "200px";
    document.getElementById("demo").style.backgroundColor = "red";
 </script>
</body>
</html>